Typical vehicle engine-cooling systems include an electric motor and an axial flow fan coupled to an output shaft of the motor. Axial flow fans typically include a hub coupled to the output shaft of the motor and a plurality of blades extending radially from the hub. The hub of the axial flow fan is typically shaped to allow the motor to be at least partially recessed into the hub to reduce the space requirement of the assembled motor and axial flow fan. A plurality of radially-extending ribs are also typically incorporated with the hub to stiffen the structure of the axial flow fan. If a ventilated or air-cooled motor is employed, the ribs may also help cool the motor by functioning as a centrifugal fan to pull cooling air through the motor.
Axial flow fans are often designed to minimize noise and vibration during operation. Some vehicle engine-cooling systems may suffer from higher than desirable levels of noise, vibration, and harshness (“NVH”) caused by, for example, motor cogging torque, axial cogging forces, torque ripple, and axial ripple forces which can excite resonant modes in the structure of the axial flow fan. To reduce the NVH caused by the axial flow fan, isolation apertures or slots are often formed in the hub of the axial flow fan.